2001 GMC Jimmy Car will not start

Our mechanic is completely stumped on this one. The original problem was that the engine was not getting the 55-60 lbs of pressure that is required for it to start. Since then, he has replaced the fuel pump, fuel injectors, intake gasket, and oil pressure switch. The vehicle still will only start if it has been primed first. Once it has been started, it will continue to be able to be started as long as it isn't turned off for an extended time.

I can tell you it is getting 55-60 lbs of pressure now. There are no codes showing up. Also, the battery checks out to be fine.

The mechanic said he did some research and found that if the vehicle is on and you remove this one relay from the fuse panel, the car is suppose to remain on. He said if the relay is removed and the engine shuts off, it is suppose to mean the oil pressure is bad. This is why the switch was replaced.

Please help find the problem. We badly need our cars working so we can get to work.